Attenuation of morphine tolerance by intrathecal gabapentin is associated with suppression of morphine-evoked excitatory amino acid release in the rat spinal cord

Assessing effects of tamoxifen on tolerance, dependence, and glutamate and glutamine levels in frontal cortex and hippocampus in chronic morphine treatment

Tamoxifen has been shown to reduce glutamate release from presynaptic glutamatergic nerves and reverse tolerance to morphine-induced respiratory depression. Changes in glutamatergic neurotransmission in the central nervous system contribute to morphine tolerance, dependence, and withdrawal. This study, therefore, evaluated effects of tamoxifen on development of analgesic tolerance and dependence, and brain glutamate and glutamine levels in chronic morphine administration. Mice implanted with placebo or morphine pellets were injected with tamoxifen (0.6-2 mg/kg) or vehicle twice daily for 3 days. Nociceptive response was evaluated in the hot plate and tail immersion tests, 4, 48 and 72 h post-implant, and following a challenge dose of morphine (10 mg/kg). Withdrawal signs were determined after naloxone (1 mg/kg) administration. Morphine increased nociceptive threshold which declined over time. At 72 h, acute morphine elicited tolerance to the analgesic effect in the hot plate test in vehicle or tamoxifen administered animals. In the tail immersion test, however, tolerance to morphine analgesia was observed in tamoxifen, but not vehicle, co-administration. Tamoxifen did not reduce withdrawal signs. In contrast to previous reports, glutamate and glutamine levels in the hippocampus and frontal cortex did not change in the morphine-vehicle group. Confirming previous findings, tamoxifen (2 mg/kg) decreased glutamate and glutamine concentrations in the hippocampus in animals with placebo pellets. Both doses of tamoxifen significantly changed glutamate and/or glutamine concentrations in both regions in morphine pellet implanted animals. These results suggest that tamoxifen has no effect on dependence but may facilitate tolerance development to the antinociception, possibly mediated at the spinal level, in chronic morphine administration.

α2δ-1-Bound N-Methyl-D-aspartate Receptors Mediate Morphine-induced Hyperalgesia and Analgesic Tolerance by Potentiating Glutamatergic Input in Rodents

BACKGROUND

Chronic use of μ-opioid receptor agonists paradoxically causes both hyperalgesia and the loss of analgesic efficacy. Opioid treatment increases presynaptic N-methyl-D-aspartate receptor activity to potentiate nociceptive input to spinal dorsal horn neurons. However, the mechanism responsible for this opioid-induced activation of presynaptic N-methyl-D-aspartate receptors remains unclear. α2δ-1, formerly known as a calcium channel subunit, interacts with N-methyl-D-aspartate receptors and is primarily expressed at presynaptic terminals. This study tested the hypothesis that α2δ-1-bound N-methyl-D-aspartate receptors contribute to presynaptic N-methyl-D-aspartate receptor hyperactivity associated with opioid-induced hyperalgesia and analgesic tolerance.

METHODS

Rats (5 mg/kg) and wild-type and α2δ-1-knockout mice (10 mg/kg) were treated intraperitoneally with morphine twice/day for 8 consecutive days, and nociceptive thresholds were examined. Presynaptic N-methyl-D-aspartate receptor activity was recorded in spinal cord slices. Coimmunoprecipitation was performed to examine protein-protein interactions.

RESULTS

Chronic morphine treatment in rats increased α2δ-1 protein amounts in the dorsal root ganglion and spinal cord. Chronic morphine exposure also increased the physical interaction between α2δ-1 and N-methyl-D-aspartate receptors by 1.5 ± 0.3 fold (means ± SD, P = 0.009, n = 6) and the prevalence of α2δ-1-bound N-methyl-D-aspartate receptors at spinal cord synapses. Inhibiting α2δ-1 with gabapentin or genetic knockout of α2δ-1 abolished the increase in presynaptic N-methyl-D-aspartate receptor activity in the spinal dorsal horn induced by morphine treatment. Furthermore, uncoupling the α2δ-1-N-methyl-D-aspartate receptor interaction with an α2δ-1 C terminus-interfering peptide fully reversed morphine-induced tonic activation of N-methyl-D-aspartate receptors at the central terminal of primary afferents. Finally, intraperitoneal injection of gabapentin or intrathecal injection of an α2δ-1 C terminus-interfering peptide or α2δ-1 genetic knockout abolished the mechanical and thermal hyperalgesia induced by chronic morphine exposure and largely preserved morphine's analgesic effect during 8 days of morphine treatment.

CONCLUSIONS

α2δ-1-Bound N-methyl-D-aspartate receptors contribute to opioid-induced hyperalgesia and tolerance by augmenting presynaptic N-methyl-D-aspartate receptor expression and activity at the spinal cord level.

The effects of gabapentin on acute opioid tolerance to remifentanil under sevoflurane anesthesia in rats

BACKGROUND

Tolerance to remifentanil during sevoflurane anesthesia may blunt the ability of this drug to reduce anesthetic requirements. Gabapentin has been shown to be effective in reducing postoperative narcotic usage, a reduction that may be associated with a reduction in opioid-induced tolerance and hyperalgesia. We sought to determine whether gabapentin might prevent the observed acute opioid tolerance (AOT) produced by remifentanil in sevoflurane minimum alveolar concentration (MAC).

METHODS

Wistar rats were anesthetized with sevoflurane and the effects of gabapentin alone on sevoflurane MAC were determined at doses of 150 and 300 mg · kg(-1). In a second experiment, gabapentin 300 mg · kg(-1) was administered before remifentanil (120 and 240 μg · kg(-1) · h(-1)). The MAC was determined before gabapentin administration and 3 more times at 1.5-hour intervals after drug administration to assess AOT. MAC was determined from intratracheal gas samples using a sidestream gas analyzer; tail clamping was used as a supramaximal stimulus. Statistical analysis was performed with the 1-way analysis of variance test.

RESULTS

Remifentanil reduced MAC (2.5 ± 0.2%) by 16% ± 5% and 36% ± 6% (120 and 240 μg · kg(-1) · h(-1), respectively, P < 0.01) with a further reduction produced by coadministration with gabapentin 300 mg · kg(-1) to 39% ± 12% and 62% ± 14%, respectively (P < 0.01 versus remifentanil alone). Gabapentin given alone at 150 and 300 mg · kg(-1) reduced MAC by 26% (both doses, P < 0.01). AOT was observed with remifentanil and characterized by a lower degree of MAC reduction, approximately 1.5 hours later (P < 0.05). However, when remifentanil was administered with gabapentin, the AOT to remifentanil was not observed (P > 0.05).

CONCLUSIONS

Gabapentin reduced the sevoflurane MAC and enhanced the MAC reduction produced by remifentanil. This enhancement may limit AOT in rats.

Role of gabapentin in preventing fentanyl- and morphine-withdrawal-induced hyperalgesia in rats

PURPOSE

This study was undertaken to examine the effect of gabapentin for preventing hyperalgesia induced by morphine and fentanyl withdrawal in rats.

METHODS

To induce hyperalgesia, Sprague Dawley (SD) rats were subcutaneously injected with fentanyl four times at 15-min intervals (60 μg/kg per injection), resulting in total dose of 240 μg/kg over 1 h, and morphine 10 mg/kg twice daily for 7 days. The effect of gabapentin was detected with behavioral tail-flick and paw-withdrawal tests.

RESULTS

Drug termination produced significant decrease in antinociception thresholds (P < 0.05 vs. saline group), indicating that the rats became sensitive to thermal stimuli. In rats that received combined treatment with fentanyl/morphine and gabapentin (25/50 mg/kg), results demonstrated that there were no significant decreases in antinociception thresholds (vs. saline group) after opioid withdrawal. Gabapentin (50 mg/kg) could also prevent morphine tolerance. The 50% effective dose (ED50) value was 12.5 mg/kg in tail-flick and 13.6 mg/kg in paw-withdrawal tests.

CONCLUSIONS

The study showed that gabapentin can significantly prevented opioid-induced hyperalgesia (OIH) induced caused by fentanyl and morphine, suggesting a role for the addition of gabapentin in the perioperative period and during chronic pain treatment as an effective drug to prevent OIH.

Gabapentin action and interaction on the antinociceptive effect of morphine on visceral pain in mice

BACKGROUND AND OBJECTIVE

Visceral pain is one of the most common forms of pain and for which new drugs would be welcome. The aim of this study was to investigate whether gabapentin inhibits induced abdominal contractions in mice and to examine the effect of its co-administration with morphine.

METHODS

A total of 96 mice received acetic acid intraperitoneally after administration of saline or gabapentin (1, 5, 10, 50 and 100 mg kg(-1)) or morphine (0.25, 0.5, 1, 3 and 5 mg kg(-1)) or a combination of morphine and gabapentin. Other groups also received naloxone. The number of writhes were counted.

RESULTS

Both gabapentin and morphine reduced writhing in a dose-dependent manner. The number of writhes was decreased significantly by gabapentin (50 and 100 mg kg(-1)) and morphine (0.5, 1, 3 and 5 mg kg(-1)) (P < 0.001). Also, the lowest dose of morphine 0.25 mg kg(-1) when combined with low doses of gabapentin significantly decreased the number of writhes (P < 0.005). The combination of a low effective dose of gabapentin (50 mg kg(-1)) with a low dose of morphine decreased the writhing by 94% as compared to the controls. The antinociceptive effect of combined administration was not reversed by naloxone.

CONCLUSION

These data demonstrated the comparable efficacy of gabapentin with morphine in visceral pain. Also, the results showed that the combination of doses of gabapentin and morphine, which were ineffective alone, produced a significant analgesic effect in the writhing model of pain. This may be clinically important in the management of visceral pain.

Central administration of selective melanocortin 4 receptor antagonist HS014 prevents morphine tolerance and withdrawal hyperalgesia

Major problem involved in treatment of chronic pain with morphine is the development of tolerance and dependence. Previous studies have demonstrated the participation of melanocortin (MC) system in the development of tolerance to antinociceptive effect of morphine. However, the impact of supraspinal MC4 receptors (MC4 R) modulation on this phenomenon and morphine withdrawal hyperalgesia remained unexplored. We investigated the role of central MC4 R in acute, chronic effects and withdrawal reactions of morphine using tail flick test. Acute intracerebroventricular (icv) administration of morphine (2-20 microg/rat) exhibited antinociceptive activity, which was antagonized by subeffective dose of nonselective MC R agonist NDP-MSH (0.04 ng/rat, icv), and potentiated by subeffective dose of MC4 R antagonist HS014 (0.008 ng/rat, icv). Isobolographic analysis revealed antagonistic interaction between NDP-MSH and morphine, and additive interaction between HS014 and morphine combinations. While chronic icv infusion of morphine (20 ng/microl/h) via osmotic pump for 7 days developed tolerance to its antinociceptive effect, its discontinuation produced hyperalgesia. Co-administration of HS014 (0.008 ng/rat, icv) with chronic morphine not only delayed the development of tolerance but also prevented withdrawal hyperalgesia. Furthermore, acute treatment with HS014 (0.008 and 0.04 ng/rat, icv) dose dependently attenuated the withdrawal hyperalgesia. This suggests the involvement of central MC4 R in the mechanism of development of tolerance and dependence following chronic morphine administration. We speculate that targeting this receptor may be a novel strategy to improve the effectiveness of morphine in the treatment of chronic pain.

Gabapentin completely attenuated the acute morphine induced c-Fos expression in the rat striatum

The neuro-anatomical sites and molecular mechanism of action of gabapentin (GBP)-morphine interaction to prevent and reverse morphine side effects as well as enhancement of the analgesic effect of morphine is not known. Therefore, we examined the combined effects of GBP-Morphine on acute morphine induced c-Fos expression in rat striatum. The combined effect of GBP-Morphine was examined by means of c-Fos immunohistochemistry. A single intraperitoneal injection (i.p.) of morphine (10 mg/kg), saline (control), co-injection of GBP (150 mg/kg) with morphine (10 mg/kg) was administered under anaesthesia. Ninety minutes after drugs administration the deeply anesthetized rats were perfused transcardially with 4% paraformaldehyde. Serial 40 mum thick sections of brain were cut and processed by immunohistochemistry to locate and quantify the sites and number of neurons with c-Fos immunoreactivity. Detection of c-Fos protein was performed using the peroxidase-antiperoxidase (PAP) detection protocol. Our present study demonstrated that, administration of GBP (150 mg/kg, i.p.) in combination with morphine (10 mg/kg, i.p.) significantly (p < 0.01) attenuated the acute morphine (10 mg/kg, i.p.) induced c-Fos expression in the rat striatum. Present results showed that GBP-morphine combination action prevented the acute morphine induced c-Fos expression in rat striatum. Moreover, this study provides first evidence of neuro-anatomical site and that GBP neutralized the morphine induced activation of rat striatum.

Intraventricular administration of gabapentin in the rat increases flurothyl seizure threshold

INTRODUCTION

We investigated efficacy of prolonged intraventricular gabapentin (GBP) infusion in the rat flurothyl epilepsy model.

METHODS

Sprague-Dawley rats, under anesthesia, were implanted with bilateral Alzet model 2001 osmotic pumps. The pumps infused GBP 80 microg/microL (3.8 mg/day) or isotonic saline control at 1.0 microL/h into each ventricle for 5 days. After 5 days of GBP infusion, seizures were induced by flurothyl dripped onto filter paper. Time to first myoclonic jerk, first partial seizure and first tonic-clonic seizure was recorded by an observer unaware of the treatment group. Determination of seizures was behavioral.

RESULTS

Data were obtained from 54 rats. First tonic-clonic seizure was at 295.8+/-58.8s (n=28) for control rats, versus 338.0+/-89.9 s (n=26) for rats with GBP in the pump (p=0.049). First myoclonic jerk occurred at 158.7+/-20.8 versus 164.6+/-33.5 s (p=0.44, n.s.). Regression of time to seizure versus weight was not significant. No animal had measurable serum levels (<1 microg/ml) of GBP. The distribution of GBP in brain was not studied, but qualitative observations of methylene blue dye installed in the pumps showed dye in periventricular white matter and also over cortex, especially ipsilaterally.

DISCUSSION

GBP instilled into the lateral ventricles by pump for 5 days delays onset of generalized tonic-clonic seizures produced by flurothyl in the rat. Time to first myoclonic or partial seizure was not influenced. Effects were not due to systemic absorption of GBP. This study provides a proof-in-principle for intraventricular therapy with AEDs.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Mechanisms of the antinociceptive action of gabapentin

Gabapentin, a gamma-aminobutyric acid (GABA) analogue anticonvulsant, is also an effective analgesic agent in neuropathic and inflammatory, but not acute, pain systemically and intrathecally. Other clinical indications such as anxiety, bipolar disorder, and hot flashes have also been proposed. Since gabapentin was developed, several hypotheses had been proposed for its action mechanisms. They include selectively activating the heterodimeric GABA(B) receptors consisting of GABA(B1a) and GABA(B2) subunits, selectively enhancing the NMDA current at GABAergic interneurons, or blocking AMPA-receptor-mediated transmission in the spinal cord, binding to the L-alpha-amino acid transporter, activating ATP-sensitive K(+) channels, activating hyperpolarization-activated cation channels, and modulating Ca(2+) current by selectively binding to the specific binding site of [(3)H]gabapentin, the alpha(2)delta subunit of voltage-dependent Ca(2+) channels. Different mechanisms might be involved in different therapeutic actions of gabapentin. In this review, we summarized the recent progress in the findings proposed for the antinociceptive action mechanisms of gabapentin and suggest that the alpha(2)delta subunit of spinal N-type Ca(2+) channels is very likely the analgesic action target of gabapentin.